Liquid natural gas sampling is governed by the standard ISO 8943 and GIIGNL LNG custody transfer handbook. Europe and other areas of the world impose additional requirements such as mandating composite sampling of transferred LNG contents, and particularly, that from tanker ship off-loading. The standard calls for a composite sampling to be collected for the duration of time that a ship is off-loading. The sampled gas stream is transferred to small cylinder sample containers for storage and comparison to continuous online analysis averages.
During transfer processing of a liquid natural gas shipping vehicle, it is desirable to obtain accurate sampling for auditing the energy content of the off-loaded LNG. This can be accomplished using known techniques such as periodic direct sampling from a takeoff vaporizer stream and/or composite sampling. While direct sampling allows for immediate analysis by an appropriate analyzer such as a gas chromatograph, it provides an accurate portrayal of the content of the LNG off-loaded from the vehicle during the entire transfer process only by extrapolating selected accumulated data. Additionally, manual direct sampling can be taken intermittently, for example, at ¼, ½, and ¾ of vessel cargo transfer. Automatic composite sampling is used to obtain particular volumes of vaporized LNG at select periodic intervals during the transfer processing. However, analysis of the typical composite sample content is available only after the transfer processing is complete.
Conventional composite sampling technology for LNG typically takes the form dome or floating piston systems. Dome systems are bladder based and require a fluid (typically water) to isolate the collection dome from the ambient environment and maintain pressure on the collected samples. The resulting extracted composite sample is subsequently transferred from the dome to sample cylinders for analysis and/or storage for later qualitative analysis. Because dome systems rely on fluid/water failure of associated water seals will contaminate the composite sample.
Floating piston samplers are of a more simple construction than dome samplers and avoid the introduction of water/fluid as the seal method, but rely on mechanical seals. Correspondingly, floating piston systems are believed to minimize the introduction of other ambient gases (e.g., oxygen) into the composite natural gas sample. However, floating piston systems include a number movable parts and seals as well as requiring a motive source to pressurize the piston. Not only do such movable parts introduce sources for sample contamination from leakage and the like, but also it is known that such systems employ relatively higher pressures to evacuate the sample chamber during cycling.